1. Field of the Invention
The invention generally relates to systems and methods for batch degassing of dielectric oils using vacuum sonication. More particularly, the invention relates to a semi-automated apparatus comprising a tank to which oil can be transferred for heating and sonication while under reduced ambient pressure in order to remove substantial amounts of dissolved gas and moisture therefrom.
2. Description of Related Art
Electronic warfare is a component of modern warfare, which, in an offensive posture, involves the use of electromagnetic radiation to inhibit an adversary from effective use of the electromagetic spectrum, thereby inhibiting the adversary's communications and surveillance activity. Techniques associated with offensive electronic warfare include such tactics as jamming, deception, and active cancellation.
Particularly for jamming activities, radio-frequency (RF) transmitters are attached to aircraft that are flown above regions where such interference is to be applied. The U.S. Navy uses a certain RF transmitter tactical jamming system known as the ALQ-99. Such a jamming system, as with many RF transmitters, includes an oil for the dielectric separation of, as well as the cooling of, transmitter components. A common dielectric oil used for this purpose is a synthetic poly-alpha-olefin (PAO).
For aircraft flown by the U.S. Navy, theses jamming RF transmitters are serviced and maintained through the use of automatic test equipment (ATE). The U.S. Navy developed a relatively standard base set of test equipment termed the consolidated automated support system (CASS). As a component of CASS in a specific configuration, a high power device test subsystem (HPDTS) was developed particularly for testing of high power equipment, such as the RF transmitters used in electronic warfare. This HPDTS, as a comprised subunit of CASS, is also termed a high power offload to CASS (HPOC).
The HPOC comprises two racks of equipment, an electronics rack for testing transmitter functions, and a liquid cooling unit (LCU). During test procedures, the RF transmitters, commonly referred to in this context as units under test (UUT), generate significant amounts of heat, which is dissipated by circulating the dielectric oil through the LCU.
It has been discovered that transmitters that have been serviced and tested in this way later exhibit increased rates of failure during use on aircraft flying at normal altitudes. The failures appear to be the result of the degradation of dielectric properties of the oil. Dielectric property degradation is generally the result of contaminants in the oil, and is often related to dissolved gases.
During testing with the HPOC the dielectric oil is exposed to atmospheric air, the components of which dissolve in the oil. In fact, the PAO in the Navy's transmitters becomes saturated with gas on exposure to air. In particular, the oil will solvate, and may become saturated with, oxygen, nitrogen, and carbon dioxide, among other gases from the air. Other dissolved gases that may be contained in the dielectric oil and which lead to dielectric degradation are break-down products of the oil, such as hydrogen and methane. As well, water either from moisture in the air or as a degradation product may also be present in the oil and reduce dielectric effectiveness.
The current LCU process executed by the ALQ-99 Test Program heats the PAO to 55° C. and then applies a 26 inch Hg vacuum for 5 minutes, removes the vacuum to circulate the PAO for 1 minute and then applies vacuum for 5 more minutes. From the PAO's saturation point of about 13.5% dissolved gases, the LCU degassing process generally results in a dissolved gas content in the PAO of about 11.5%-12%, a removal of only about 1-2% of the dissolved gases. For optimum operation, however, the ALQ-99 RF transmitter requires the PAO installed therein to have a total dissolved gas content of less than 1.75%.
There are several known methods for removing dissolved gas from liquids. A first and relatively simple method is to apply a high vacuum to a closed space adjacent to the surface of a batch oil. Through an equilibrium-driven process, the dissolved gases will eventually be removed from the oil to the vacuum. This equilibration process will generally take a long time, however. Such a process is the method used by the existing LCU, and without modification, is insufficient to meet the current needs of the ALQ-99 and other dielectric oil applications.
A modification of the first method increases the rate of oil degassing by maximizing the surface area of the oil so that dissolved gases in the oil have a lesser distance to travel to escape from the surface of the oil, resulting, for any given volume of oil, in less time to degas. Increased surface area can be accomplished by creating a thin film of oil, such as by spraying the oil onto a surface, for example, a spinning disk, filters, or by atomizing the oil into a fine mist. A principal disadvantage of this method is based in the balance that must be struck between the size of the apparatus in which the oil's increased surface area is created and the volume throughput of oil through the apparatus. Increasing oil surface area either requires a time consuming repetition on multiple small volume samples of the bulk oil of a process such as spraying or atomizing, or requires a large apparatus in which larger volume samples of oil can be sprayed or atomized. Commercial products using this increased-surface-area method are available, but are primarily used in the electric power industry, where size is not a significant factor since the apparatuses are generally stationary and associated with large commercial power plants. These commercial apparatuses are too large for use shipboard or in a mobile analytical laboratory, either of which are preferred circumstances for testing and maintenance activities associated with RF transmitters used on U.S. Navy aircraft.
Further modification to the above-described methods involves applying ultrasonic pressure waves to the oil, while simultaneously applying a high vacuum to a closed space adjacent to the oil. The sonication forces dissolved gases to coalesce into bubbles, the gas bubbles then rise to the oil surface and burst releasing the gas to the tank head space, from which the vacuum system removes the gases. The gas bubbles formed by coalescence travel much faster through the oil than do the uncoalesced, dissolved gases in the first method.
Systems for utilizing a sonication degassing method for transformer oils is described in both Soviet Union patent publication SU929150, filed Jan. 9, 1980, published May 23, 1982, naming as inventors, Sukhanov et al. (the '150), and Russian patent publication RU2186095, filed Aug. 14, 2001, published Jul. 27, 2002, naming as inventors, Shved et al. (the '095). The '150 describes a system in which transformer oil is pumped under relatively high pressure from the transformer into a vacuum tank in a stream that impinges on an acoustic resonator and thereby creates ultrasonic vibrations in bulk oil collected in a processing tank. The '095 describes a system in which oil is recirculated through two vacuum sonication tanks, and is repeatedly being filtered for the purpose of accelerating the cavitation process. During recirculation, the oil is atomized as it enters each tank. The described method includes a recirculation process whereby processed oil is returned to the transformer and mixed therein with unprocessed oil, so that the method applies a combined batch and continuous mode of operation. The result of processing is indicated as removing gas from oil having a starting gas content of about 1% to produce oil having a gas content of about 0.1%.